The present invention refers to a blending silo, and in particular to a blending silo of the type having a silo vessel with conical bottom ending in a central outlet.
A blending silo of this type can be designed as gravity mixer or as circulation mixer. For both applications, it is known to equip the silo vessel with internal fittings by which a high mixing degree of bulk material during a single run is attained, i.e. a good homogenizing of different and usually sequentially charged bulk material, or--in case of circulation mixers--the number of and thus the mixing time for circulations is kept to a minimum.
DE-AS 1,298,511 and EP-Al-0,060,046, both describe a blending silo with its interior being divided in several compartments by vertical sheet segments which radially extend from the outer vessel wall to the center axis. These sheet segments are provided with respectively graduated upper edges to allow successive filling of the compartments when suitably positioning the feed opening, through overflow of one compartment after another Instead of the otherwise occurring purely horizontal arrangement in layers, a vertical pre-mixing effect is thereby created, which however is dependent on the charge quantity.
German patent no. DE-PS 22 19 397 discloses a blending silo in form of a circulation mixer in which the central riser pipe is surrounded by an essentially shorter pipe which defines a first annular space with the riser pipe and a second annular space with the vessel wall of the silo and its conical bottom. During circulation or during withdrawal of bulk material, the sinking speed of bulk material in both annular spaces varies so that bulk material fractions from different level can be blended in the outlet area. German publication DE-OS 30 29 393 describes a gravity-circulation mixer which is based upon a similar principle; however, the circulation is not attained via a central pipe, but by means of a vertical riser pipe which extends externally of the silo vessel.
Common to all these known blending silos is their drawback that the internal fittings are subjected to considerable static and dynamic loads which require complicated calculations with regard to stability and dimensioning and usually exclude a subsequent modification of a silo vessel. Even though conventional blending silos are dimensioned based upon mass flow conditions, deposits of bulk material are encountered which at least complicate a cleaning of the silo vessel. However, users increasingly demand a capability for easy cleaning of the silo vessel before charging the blending silo with a different type of bulk material.